The invention relates to a method for controlling uplink access transmissions in a radio communication system, especially in a mobile communication system.
In radio communication systems, signals are exchanged between radio terminals and base stations via a so called radio interface or air interface. Such radio terminals are for example mobile or stationary user equipments (UE). On the other side, base stations (NB—Node B) are access stations associated with a land based communication network. Examples of known radio communication systems are second generation digital mobile radio communication systems like GSM (Global System for Mobile Communication) based on TDMA (Time Division Multiple Access) and providing data rates up to 100 kbit/s, or third generation digital mobile radio communication systems like UMTS (Universal Mobile Telecommunication System) based on CDMA (Code Division Multiple Access) with data rates up to 2 Mbit/s.
In such communication systems, for the establishment of specific services it may be necessary or at least desirable to know the number of all user equipments or only those with particular characteristics which are currently located in a cell of a base station. One method of implementing such a process in case the network has no prior knowledge of the user equipments' presence in the cell, is for the network to request the user equipments to respond to the request on a contention based common uplink access channel, for example on a random access channel (RACH) known from the GSM and UMTS systems mentioned above. This enables the network to count the number of responses received from the addressed user equipments until a predetermined number (threshold) is reached, or, if less than the threshold number of user equipments are situated in the cell, until a predetermined time interval has lapsed. As a negative consequence, if a large number of user equipments is situated in the cell, this could lead to a signalling overload situation in the common uplink signalling channel, with the consequence of risking disruptions to the network's normal operation in the cell.
A particular example of where this problem may occur is the so called MBMS (Multimedia Broadcast/Multicast Service) service provision which is currently standardised to for UMTS and GSM/GERAN. Radio bearers for multicast MBMS services are set up within a cell only if there are user equipments present in the cell with the particular MBMS service activated. If the number present is below a threshold value, individual radio bearers are established to each of the user equipments (point-to-point bearers) whilst if the number exceeds the threshold value, a single multicast radio bearer is established to serve all user equipments present in the cell.
Consequently, the network needs to know whether the number of user equipment present in the cell exceeds the threshold and eventually, if less than the threshold number are present, the identities of the user equipments in order to build up individual bearers. For user equipments which are in the so called UMTS connected mode, the network knows the total number of user equipments present in the cell with the particular MBMS service activated without communicating directly with the user equipments. In contrast thereto, for user equipments which are in an unconnected state, e.g. in a so called idle mode or URA_PCH state, the network needs to signal a request to the user equipments, e.g. by paging or control channel signalling, and the user equipments will need to respond on the RACH (Random Access Channel) common uplink signalling channel. Counting of user equipment responses should be completed within a predetermined time interval, and a decision whether or not a single multicast radio bearer is established based on the result of the counting.
In current radio communication systems, access signalling on contention access channels like the RACH is controlled individually by the user equipments by selecting an initial delay period after which, when lapsed, the user equipment transmits access signals. This initial delay may be uniformly distributed, like in GSM, or effectively negative exponentially distributed, like in UMTS. Such initial delay distributions are well suited for responses to selective paging of individual user equipments, where simultaneous access attempts from a large number of user equipments are unlikely. However, they would not be efficient in preventing the occurrence of signalling overload in contention based access channels when a great number of user equipments is paged simultaneously as would be the case for a group page for establishing MBMS services.